1. Field of the Invention
This invention relates to a tank internal pressure-detecting device for detecting pressure within a fuel tank of an internal combustion engine, and more particularly to a tank internal pressure detecting device of this kind, which has a function of making a so-called zero point correction of an output from a sensor for sensing the pressure within the fuel tank.
2. Prior Art
Conventionaly, there has been widely used an evaporative fuel-processing system for internal combustion engines, which comprises a fuel tank, a canister having an air inlet port provided therein, a first control valve arranged across an evaporative fuel-guiding passage extending from the fuel tank to the canister, and a second control valve arranged across a purging passage extending from the canister to the intake system of the engine.
A system of this kind temporarily stores evaporative fuel in the canister, and then purges the evaporative fuel into the intake system of the engine.
Whether a system of this kind is normally operating can be checked, for example, by bringing the evaporative emission control system into a predetermined negative pressurized state, measuring a change in the pressure within the fuel tank (tank internal pressure) with the lapse of time after the evaporating emission control system has been brought into the predetermined negatively pressurized state, and determining whether the system is normally operating, from the measured tank internal pressure, as proposed by U.S. Ser. No. 07/942,875 assigned to the assignee of the present application.
The abnormality-determining system according to the earlier application includes a third control valve provided at the air inlet port of the canister, for closing and opening the same, tank internal pressure-detecting means for detecting pressure within the fuel tank, a first pressure change rate-detecting means for detecting a rate of change in the pressure within the fuel tank by controlling opening and closing of the first control valve, negatively-pressuring means for setting the evaporative emission control system to a predetermined negatively pressurized state by controlling opening and closing of the first to third control valves when the engine is operating a second pressure change rate-detecting means for detecting a range of change in the pressure within the fuel tank by closing the second control valve after the predetermined negatively pressurized state has been established, and abnormality-determining means for determining abnormality of the evaporative emission control system based upon results of detection by the first and second pressure change rate-detecting means.
According to the above-mentioned abnormality-determining manner, the rate of change in the pressure within the fuel tank is a rate of change of the pressure in the fuel tank which occurs in the direction toward a positive pressure side (positive pressure side) due to generation of evaporative fuel within the fuel tank, while the rate of change in the fuel tank pressure is a rate of change in the fuel tank pressure which starts from a negative pressure state by closing the second control valve, after the evaporative emission control system been brought into the predetermined negatively pressurized state (the pressure within the fuel tank and that within the canister have been made negative) by opening the second control valve (negative pressure side change). The two kinds of pressure change rates detected by the first and second pressure change rates detecting means are compared with each other to detect an abnormality in the evaporative emission control system (i.e. an mount of evaporative fuel leaking from the system).
However, according to the abnormality-determining manner, the output value of the tank internal pressure sensor can vary due to manufacturing tolerances, aging, etc. If the output value of the sensor detected at the point of inflection, i.e. at the point at which the output value changes from the positive pressure side to the negative pressure side or vice versa (zero point) deviates from the actual point of inflection, it is impossible to discriminate whether the pressure within the fuel tank is changing toward the positive pressure side or toward the negative pressure side, resulting in an inaccurate abnormality determination based upon the leakage mount from the evaporative emission control system, etc.
Further, if the zero point of the tank internal pressure sensor deviates from the proper one, the interior of the fuel tank, etc. can be excessively negatively pressurized during the negative pressurization wherein the interior of the evaporative emission control system is negatively pressurized.